When a tank or other closed space to be ventilated by pressure relief valves of the kind according to the present invention is loaded with a given volume of liquid per time unit, the pressure relief valves open when the gas pressure within the tank reaches a predefined opening pressure and, upon opening of the pressure relief valve, a volume of gas per time unit which corresponds to the product volume loaded per time unit plus any evaporation from the liquid surface in the tank is then to be blown out, hence the designation “pressure relief valve”.
The blown-out volume per time unit depends primarily on the flow resistance of the valve and on the pressure prevailing within the tank. The maximum blown-out volume per time unit minus any evaporation, which may occur without the pressure within the tank exceeding a given safety threshold value, Pmax, is the designated capacity of the pressure valve.
The capacity of the pressure relief valve determines the maximally allowable loaded volume per time unit. When, upon interruption of the loading, the pressure within the tank drops to, or below, a certain value, i.e. to or below the closing pressure of the pressure relief valve, the pressure relief valve close.
The closing pressure of the pressure relief valve typically is lower than the opening pressure of the pressure relief valve.
Most prior art pressure relief valves are configured such that the closing pressure of the valves is considerably lower than the opening pressure of the valves. By this, most prior art pressure relief valves ventilate any boiling off or evaporation from products like petroleum products or chemicals in fundamentally the same manner as the valves ventilate during loading of the cargo space. This is considered undesirable as most prior art pressure relief valves cannot maintain a relatively constant excess pressure in a cargo space to be ventilated. In fact, most prior art pressure relief valves subject the space to be ventilated to large pressure fluctuations. The large pressure fluctuations cause wasteful and unnecessary escape of often harmful gas.
Another use of pressure relief valves applies when, for safety reasons, it is required to supply an inert gas, such as nitrogen, to the free ullage of a container above the product being transported. For instance, the product may be an inflammable liquid or a material, such as oranges and lemons that gives off inflammable gasses, following storage. The inert gas is automatically kept at a predetermined pressure that may be configured to lie above the atmospheric pressure, but below the opening pressure of the pressure valve. In this case any rise or a drop in the temperature of the surroundings will give rise to a corresponding change in the pressure of the inert gas within the container, whereby the pressure valve must open.
In prior art pressure relief valves, it has been found that the difference between the opening pressure of the valves and the closing pressure of the valves, due to the design of the pressure valves, is too large for the pressure valves to be optimally suited for use in the transport of certain materials. This has been found to be particularly problematic in large capacity pressure relief valves.
U.S. Pat. No. 5,060,688 A discloses a pressure relief valve for pressure equalization between a closed space and the surrounding atmosphere. The valve includes a valve housing with an upwardly facing blow-off opening. A drop-shaped valve body is arranged in the blow-off opening. The valve body is connected to a lifting disc which has a larger area than the valve seat and the lifting disc is surrounded by a free passage slot defined by the lifting disc and the wall of the valve housing. According to the reference, advantageous opening and closing characteristics can be obtained as the free area of said free passage slot increase when the valve opens. The increase in free passage of the slot is obtained by complex geometries of the moulded wall defining the flow passage of the pressure relief valve.
US 2004/0177883 A discloses a pressure relief valve. The valve includes a valve body and a lifting plate that has a surface area A2 oriented transversally of the flow passage that exceeds the cross sectional area A1 of the flow passage at the valve seat. The pressure relief valve is configured such that, at any time, the container to be ventilated is in communication with the area above the lifting plate. According to the reference, a magnet and a magnetisable body contributes to defining an abutment force Fc, of the valve body against the valve seat in the closed state of the pressure valve. By this, an increased opening pressure of the pressure relief valve is obtained.
U.S. Pat. No. 5,873,384 A discloses pressure relief valve including a valve housing and a drop-shaped valve body with a conical valve surface configured to cooperate with a conical valve seat at an upwardly directed blow-off opening of the valve housing. The drop-shaped body is connected to a lifting disc in the housing. The inner configuration of the housing provides an annular passage around the lifting disc which is smaller in area when the lifting disc and the drop-shaped body is in its uppermost, fully open positioning as compared to its lowermost, closed positioning.
The above references are incorporated herein in their entirety.
The present invention seeks to provide an easy to produce pressure relief valve of the kind described above, wherein the difference between the opening pressure and the closing pressure can be maintained at a minimum even for large capacity pressure relief valves. In particular, it is an object of the present invention to set forth an easy to produce pressure relief valve configured to avoid abrupt closure of the valve as a result of reduced pressure in the space to be ventilated.
This is obtained by the pressure relief valve according to the present invention and as described in the appended claims.
By the embodiments featured in the dependent claims, a particularly convenient configuration of the characteristic curve of the pressure valve during the course of the closing the pressure relief valve is obtained.